David Miliband enters the home stretch of the Labour leadership race claiming the support of more than half the shadow cabinet amid speculation that the contest will be so close it could be decided by second and third preferences.

All candidates were attempting to move the contest away from the "soap opera" of sibling rivalry and the politics of old versus new Labour after interventions from senior party figures.

Ballot papers will be sent to Labour party members with up to 80% expected to return them within the week. The result will be announced on the eve of the Labour party conference on 25 September.

The shadow foreign secretary, bookies' favourite, made a peace offering to his brother Ed Miliband by saying he would serve in a shadow cabinet led by his younger sibling. Ed previously said he would do the same.

The pair attempted to distance themselves from the Brown-Blair legacy after interventions from Lord Mandelson and ahead of the publication of Tony Blair's memoirs today. There is speculation that Blair will offer a veiled backing for David Miliband in an interview with Andrew Marr to be broadcast by the BBC.

In a letter to party members, 11 of the 21 frontbench shadows who will vote in this month's election declare David Miliband the best candidate to return Labour to power. The signatories include shadow chancellor Alistair Darling, shadow home secretary Alan Johnson and shadow justice secretary Jack Straw.

The letter says: "We have sat in cabinet with other candidates but we believe it is David Miliband who would best lead our party. He has the values, vision and strength to lead us to victory. David has shown a determination to honestly understand the reasons why Labour lost but has done so without undermining our many achievements. He understands that we need to rebuild our party organisation as a movement for change.

"It is David's focus on the future rather than the battles of the past that single him out from the pack. He knows that Labour will only win again if we focus on the challenges facing our country and become the change that Britain needs."

The letter is also signed by Bob Ainsworth, Douglas Alexander, Ben Bradshaw and Liam Byrne. It ends: "We know David Miliband. We have worked beside David Miliband. He has what it takes to lead and to win. That is why we urge Labour members to vote for David as next Labour leader."

Ed Miliband's campaign was claiming that he would benefit from the voting system and that they had identified people backing Ed Balls who had promised their second vote to Ed Miliband. While David Miliband is expected to win the first round, there are predictions that once other candidates are eliminated and their second and third preferences counted, he will face a close challenge from his brother.

Ed Miliband made a new pitch to Labour's left, suggesting the system for holding ballots to abolish grammar schools should be reviewed, potentially making it easier to end selection.

The shadow energy secretary told the Evening Standard he was not seeking to close grammars .

Balls, the shadow schools secretary, labelled the contest a "soap opera" between the Milibands that was drowning out policy debate. Andy Burnham criticised senior Labour figures for portraying the contest as "a battle between old and new Labour". The shadow health secretary said the party had to move on from "the era of self-indulgent factionalism".